1. Technical Field
This invention relates to liquid crystal displays (LCDs), and, more particularly to a method for etching indium-tin-oxide (ITO) electrodes and lines using self-assembled monolayers. More specifically, it relates to an etch barrier for indium-tin-oxide, a process for forming an etch pattern on a layer of indium-tin-oxide, and a method for preparing a stamp for transferring an etch barrier material.
2. Background Art
Flat panel displays have become increasingly important in computer industries and in other industries where the display of information is important. These types of displays provide unique opportunities for lowering the weight, size and eventually the cost of displaying information. Thin film transistor-liquid crystal displays (TFT-LCDs) seem to hold the most promise as the technology which will eventually be utilized in almost all practical flat panel displays. Considerable success has been achieved in small size color flat panel displays for consumer electronics as well as larger size color flat panel displays used in notebook or laptop computers.
A TFT-LCD device is made up of two substrates. One contains color filters and the other has TFT arrays. Since ITO is transparent and conductive, it is employed as an electrode or as a conductor on both substrates. It is important to be able to make a desired pattern of the ITO in modern displays. Conventionally, a photoresist and its lithographic processes are used to pattern the ITO. There has been a long felt need for a simplified process to pattern ITO, which will lower the manufacturing cost.
An alternative approach to lithography has been published by Kumar and Whitesides in Appl. Phys. Lett. 1993, 63, 2002-2004. In this process, known as microcontact stamp lithography, stamps are fabricated by casting a replica in poly(dimethylsiloxane) (PDMS) of a master with a negative of the desired pattern. The PDMS stamp is inked with hexadecanethiol, and transferred to gold substrate by transient contact between the stamp and the gold substrate.
The thiol covalently binds to the gold substrate, preventing spread of the bulk liquid phase and thus confining the transferred monolayer to the region of contact. The presence of these thiols allows subsequent lithographic processing of the gold using a cyanide/oxygen etch that selectively removes gold not protected by a monolayer of hexadecanethiol. But its scope is limited to special subset of useful materials, i.e., thiols and gold. In PCT WO 97/06012 and WO 97/06013, Biebuyck and Michel described a detailed method for application in electronic devices.
Self-assembled monolayers (SAMs) are thin organic monolayers of molecules having end groups selectively adsorbed to solid surfaces (R. G. Nuzzo and D. L. Allara, Journal of American Chemical Society vol. 105, pp. 4481-3, 1983). The solid is usually an inorganic material or a metal. SAMs of thiols adsorbed on gold have ordered structures, and their methylene (CH2) groups have all trans configurations. SAMs of n-octadecylsilane were prepared on glass surfaces using hydrolyzed n-octadecyltrichlorosilane (J. Sagiv, Journal of American Chemical Society, vol. 102, pp.92-98, 1980) or hydrolyzed n-octadecyltrimethoxysilane (C. R. Kessel and S. Granick, Langmuir, vol 7, pp. 532-538, 1991).
This invention is directed to new SAM materials, of for example, an alkylsilane, as etch barriers for indium-tin-oxide (ITO). The invention is also directed to a new method for applying SAMs to an ITO surface. The method can be used for both multi-domain LCDs and single-domain LCDs. It is applicable to both high density and low density liquid crystal displays. It can be used for direct view active matrix liquid crystal displays (AMLCD), and it can also be used for liquid crystal light valves, of both the transmissive and reflective types.
In one embodiment of the present invention, a self-assembled monolayer, such as an alkylsilane, is used as an etch barrier for ITO. The etching of ITO by a common ITO-etch solution is prevented and a desired pattern can be formed by using n-octadecylsilane SAMs. The previously described stamping method may be used to apply SANs on ITO.
Thus, in accordance with the invention, an etch barrier for indium-tin-oxide comprising an alkyl-silane self-assembled monolayer on a layer of indium-tin-oxide, the indium-tin-oxide being disposed on a substrate. The alkylsilane is selected from the group consisting of n-dodecylsilane, tetradecylsilane, n-hexadecylsilane, n-octadecylsilane, alkylchlorosilanes, alkylmethoxysilanes, and alkylethoxysilanes. These materials are at least partially hydrolyzed or fully hydrolyzed, preferably in an alcoholic solvent, prior to being applied to the indium-tin-oxide. The concentration of alkylsilane in the alcoholic solution is in the range of 0.01 5.0 percent by volume, and preferably 0.1-0.5 volume percent by volume. Preferably, the alkylsilane is n-octadecyltrihydroxysilane. It may be formed from n-octadecyltrimethoxysilane that has been hydrolyzed.
The invention is also directed to a process for forming an etch pattern on a layer of indium-tin-oxide, comprising applying a self-assembled monolayer onto the indium-tin-oxide, such as an alkylsilane; and etching the indium-tin-oxide to form the etch pattern. The alkylsilane may be one of the materials mentioned above. The process may further comprise inking a stamp with a polar SAM precursor solution; and transferring the solution onto the indium-tin-oxide.
In a specific embodiment of the present invention, the process of applying SAMs onto ITO substrates comprises the steps of applying ink to a patterned stamp with an alcoholic solution of SAM precursors, and transferring the inked solution onto ITO substrates. In a typical embodiment, n-octadecyltrimethoxysilane (OTMS) is hydrolyzed to n-octadecyltrihydroxysilane in a solvent that is a mixture of isopropanol and water for 7-30 days. The concentration of OTMS is in the range of 0.1-1.0 volume %. Either partially or fully hydrolyzed solutions may be used. The solution is applied onto ITO and baked at 85 degree C. for 30 min and the n-octadecylsilane SAM is formed on ITO. ITO underneath the SAMs can not be etched with an acid, in particular 3.5% aqueous oxalic acid, at 35 degree C. for 5 min, while ITO (500 angstrom thick) which is not covered with a SAM is completely etched.
In accordance with the invention, the process of preparing stamps includes, if necessary, modifying the stamp surface with mild oxygen plasma treatment to make the surface wettable with the polar SAM precursor solution.